Familiars

People have already got into how powerful these bonuses are, and also how uneven, so I thought I'd mention something else that hasn't brought up: Advantages in a 3.5e rules environment.

5e D&D uses Advantages (and I think Disadvantages?) very often, usually where you would expect to see a bonus or penalty in 3.5e. It uses the fact that having Advantage is a significant change in someone's abilities, but it doesn't a) stack and b) doesn't break perceived power balance. If you have Advantage against a powerful boss monster you need a 20 to hit, it'll help, but not much. What Advantage works best with is making sure you don't screw up minor tasks. If you need an 11 to hit a goblin, for example, it'll turn your 50% chance to hit into a 75% chance, the equivalent of a +5 bonus! This makes Advantage seem significant, without being the equivalent of a Balor-killing +5 sword just for, say, flanking someone.

So back to your familiar rules. The trouble with using Advantage in 3.5e, is that the rules ecology doesn't exist to properly balance the bonus you get. If you have a fighter in 5e that somehow gets Advantage on all his longsword attacks, that's obviously great, but if he flanks an enemy with a fellow fighter who doesn't have Advantage on all his attacks, his edge disappears. Now both of them just have Advantage from flanking, because it doesn't stack. Further, if something the enemy does gives both fighters a Disadvantage, they both take the hit, because Advantage is always countered by Disadvantage, even if you have Advantage from two different sources. But if you bring these rules into a game system which has no other use of them, you have an Advantage which never becomes redundant because of doubling sources, and never becomes cancelled. These skill Advantages are way more powerful than they would be in 5e, and they would already be super great in 5e.

On a more subtle note, it's easy to think applications of Advantage in checks are all equal. But 5e Dungeons and Dragons was very carefully balanced, and its rules take into account how those skills and checks might come about. So, for example, Advantage on an attack roll is generally fine because usually you aren't trying to hit things with way lower AC than you have...uh...THAC0. Whatever they call it in 5th edition. But because of the way Advantage works, getting Advantage on Hide rolls is more game-breaking. If someone has a high AC, you are going to swing at them either way, because that's a fighter's job. But if your plan involves sneaking past a guard, and you need a 20 to succeed, you don't do that plan. Sneaky stuff skill rolls are usually about not embarrassing yourself, while attack rolls are about trying to achieve difficult aims. Which means, the way Advantage works out, having Advantage on Hide is very game changing. If you look into the math on it, having a wizard with a universal Advantage on their Hide and Move Silently with make them potentially better than Rogues, especially at low levels. That's a Pretty Bad Thing.

Yeah, that's a big factor. Rolling twice on your d20 is a pretty big bonus, it works out at something like +4 on average (run the numbers) but it also makes it very unlikely that you'll roll very low (you now have a 1 in 400 chance of getting a 1, rather than 1 in 20, but a 39 in 400 chance - almost 1 in 10 - of getting a 20) - it's particularly brilliant for attack rolls and saving throws where natural 1s are an automatic failure, for this reason; you basically won't get them any more.

However if this ability doesn't stack with other similar abilities which are very common, then it becomes way less useful. You'd kind of need to know that before you can judge these - otherwise you might find that you've actually downgraded the power of the familiars a lot. (Indeed, if you changed Alertness to do the same, for example, you've made the Raven useless since familiars already grant Alertness!)

But yes, the Hare is way better than the Cat, and most things are way better than both of them. The Raven is already one of the best familiars in the game because it can speak, so people take it even because +3 to Appraise is nigh-useless. Giving it "roll twice" on Spot and Listen makes it rather awesome. The Bat is already great because it has blindsense - now it gives you blindsense too!

Also remember that in 3.5 you can get a familiar with a feat. Now OK, Iron Will is a lame feat but the Hedgehog gives you something 2-3 times better than that, plus a +1 to NA, plus the fact that all familiars grant Alertness, oh and also now you have a magic Hedgehog following you around who can use things like Imbue Familiar with Spell Ability for you, or whom you can cast Chill touch on, then have them roll into a ball so that you can throw them at an enemy to deliver spiky necromantic death!

So what I would suggest is, figure out how powerful you actually want these abilities to be. Familiars are already pretty useful, so most would say that they don't need the boost - maybe some of the lamer ones like the cat, but not the raven or the rat or the weasel. If you do want to boost them - to what point? What is "going too far"? I'd have expected them to be worth about a feat or so (and currently, they are, they're like a good feat but one which is a bit of a nuisance to use really well) but if you want to bump them up, have a clear idea of where you want them to be. Then, try and make them all conform to it. Right now these are sort of a bit all over the place.